This invention relates to a rigid polyvinyl chloride pipe having smooth inner peripheral surfaces, more particularly, to a pipe having a structure adapted for large-diameter design.
Conventional rigid polyvinyl chloride pipes are produced by extrusion molding and they have smooth inner and outer surfaces with uniformity in wall thickness along the entire length. Such "straight" rigid polyvinyl chloride pipes having various advantages including fairly good weather resistance due to the nature of the constituent material, easy availability of the material and ease in manufacture. Because of these advantages, the pipes have been produced and sold on the market in large quantities for many years. However, most of the commercially available straight, rigid polyvinyl chloride pipes are small in diameter.
In spite of their small diameter, the conventional straight, rigid polyvinyl chloride pipes have to be formed in comparatively large wall thickness in order to satisfy the requirements for high resistance to pressure and impact. To this end, the resin material must be used in a large quantity but then not only does the pipe weight increase but also the increase in production cost is inevitable. Further, if one wants to produce a pressure-resistant straight pipe of large diameter, the wall thickness must be increased but, again the increase in the use of resin material, the pipe weight and production cost is inevitable. As long as the straight geometry is adopted, it has been impossible to eliminate those disadvantages from the conventional straight pipes.
There is another problem that is peculiar to rigid polyvinyl chloride; that is, two sheets of rigid polyvinyl chloride cannot be completely bonded together even if they are molten and the joint will easily delaminate or separate.